Various embodiments described herein relate to light sources, particularly luminaires, for providing special lighting patterns. These embodiments have particular, but not exclusive, usefulness in providing what is known in the art as “batwing” lighting patterns.
In many illumination systems, targeted areas to be illuminated are much larger than an emitting area of the light sources. Many artificial light sources emit light in an approximately Lambertian distribution. When illuminated from above by such a source, flat targeted areas such as roads, floors, or a work surface cannot be illuminated uniformly without modifying the intensity distribution of the light source. When a light source with Lambertian intensity distribution illuminates a flat surface from above, the intensity on that surface will be greatest directly under the light source, and will decrease monotonically for points on the surface farther away. A “batwing” distribution, conversely, reduces the intensity at nadir (directly under the light source) and increases the intensity at angles up to some maximum angle, such that the surface is illuminated substantially uniformly for angles less than the maximum angle. Batwing radiation patterns or light distributions can exist in several forms: one-dimensional (1D) batwings have a batwing shape only to the sides (e.g. East-West direction) and are often used with linear lighting. Two-dimensional (2D) circular batwing distributions create a batwing “cone” of light, illuminating evenly in all radial directions to achieve a disc-shaped area of uniform illumination on a flat surface. 2D square or rectangular batwings create a batwing “pyramid” of light, illuminating evenly in both North-South and East-West directions to achieve a square- or rectangular-shaped area of uniform illumination on a surface, substantially filling in dark corners between luminaires arrayed in a square or rectangular array on a ceiling. Frequently luminaires with batwing distributions can provide the desired uniformity of illumination at a greater luminaire-to-luminaire spacing than with Lambertian luminaires, meaning that fewer luminaires are necessary to illuminate the desired area, saving cost. In addition, the nadir suppression involved in a batwing distribution means minimum lighting levels can be met across the surface without far exceeding that minimum level at the nadir, which would unnecessarily waste energy.
A downward-facing light source with Lambertian light distribution has luminous intensity that is proportional to the cosine of the angle from nadir (the downward-facing direction). A Lambertian light distribution is represented in polar coordinates in FIG. 1. When a flat surface such as a floor is illuminated by a Lambertian light distribution, the illuminance on the floor is greatest at nadir (directly under the fixture) and decreases monotonically for points on the floor away from nadir. The central brightness is often referred to as a “hot spot” in the lighting industry, and is generally undesirable. By definition, the Full Width at Half Maximum (FWHM) of a Lambertian distribution is 120 degrees. In the lighting industry, the term “Lambertian” is also frequently used to refer to light distributions with similar quality but of different widths. That is, distributions that have a peak at nadir, and monotonically decrease at higher angles are often called Lambertian. In one example, a Gaussian distribution with FWHM of 80 degrees will often be called “Lambertian” in the lighting industry. Lambertian distributions are not batwing distributions.
For a single ceiling luminaire, which is small compared to the ceiling-to-floor distance, to uniformly illuminate a specified width across a flat surface such as a floor, it generally must emit light in a batwing distribution whose luminous intensity is inversely proportional to the cube of the cosine of the angle from nadir for angles less than the maximum angle. This theoretical distribution can be represented by the solid curve in FIG. 2, in which no light extends beyond the maximum angle. In practice, multiple luminaires are generally used to illuminate a surface such as a room, warehouse, or roadway, and it is desirable to have some overlap, or crossfade, between the light distributions emitted by each light source. Thus a practical batwing light distribution often has some light extending beyond the maximum angle, as illustrated in the dashed curve of FIG. 2. The sharp “peaks” of the light distribution in the solid curve are also disadvantageous because they can be noticeable to a viewer, and are hard to create in practice. The dashed curve of FIG. 2 shows more practical rounded peaks in the light distribution.
In practice, it is acceptable to have some level of variation of the illuminance on a surface. For various lighting applications, an illuminance variation of about 50%, 20%, 10%, 5%, or less may be acceptable across the surface of interest when illuminated by an array of luminaires. Because the specified level of variation allows for some deviation from ideal conditions, the batwing diffuser is allowed to have a light distribution that doesn't exactly follow the 1/cos3 distribution. This imperfection is illustrated in central portion of the dashed curve in FIG. 2.
Real-world lighting situations often include extra light, reflected from floors, ceilings, and/or other objects in the illuminated space. These reflections may be random in nature, and thus may increase the uniformity on the flat surface beyond the uniformity provided by the array of luminaires alone. This may also allow the luminaire's light distribution to deviate further from the ideal 1/cos3 distribution and still achieve a desired level of uniformity on the flat surface.
In lighting, batwing light distributions different from the typical inverse cosine cubed shape are also used. These may be desired, for example, in a library or store, in which it may be desired to illuminate vertical surfaces of shelves holding books or items. For these and other lighting applications, a degree of nadir suppression may be desirable that is greater or less than the typical inverse cosine cubed shape.
Other non-Lambertian lighting distributions are also beneficial for specific applications in lighting. Wall-grazer and wall-wash distributions seek to evenly illuminate a wall from a lighting fixture placed above and some distance from the wall. Narrow, collimated, or spot distributions seek to confine light in a narrow angular spread to provide very localized illumination. Asymmetric distributions may provide more light to one side of a fixture than the other side, for example to evenly illuminate a floor from a wall-mounted fixture.
Some lighting distributions seek to reduce glare, or light emitted at high angles, usually in the range of 65-90 degrees from nadir. Such light can reflect from computer monitors and reduce visibility. For office environments in the United States, ANSI/IESNA RP-1-04 suggests limits on light emission into these angles.
High-efficiency LED lighting is being increasingly adopted. Typical LED light sources emit light into a Lambertian distribution with a Full Width Half Max (FWHM) of approximately 120 degrees. Although LEDs with many other light distributions are available, many cost-effective LEDs sold for general lighting are of the 120 degree Lambertian variety. In many luminaires, a simple planar diffuser (such as a microstructured, holographic, or volumetric diffuser) is used to diffuse the LEDs, hiding their appearance from viewers and smoothing the surface appearance of the luminaire. These diffusers may not produce batwing distributions. Rather, they typically give Lambertian distributions of various widths (most typically about 80 to 120 degrees).
Conventional diffusers known in the art come in many varieties including volumetric, microstructured, holographic, and kinoform diffusers. Conventional diffusers can range in their diffusion strength from very light (in which an object viewed through the diffuser may be blurred but recognizable to very heavy (in which the diffuser may appear milky white and translucent, and objects may not be recognizable when viewed through the diffuser). The strength of the diffuser is sometimes characterized by illuminating one surface of the diffuser with a collimated light source such as a laser from a direction normal to the diffuser's surface, and goniometrically measuring the light output from the opposite surface. The diffuser is then defined by the Full Width at Half Maximum (FWHM) of the angular spread of light emitted from said opposite surface. Thus a 30-degree conventional diffuser when illuminated by a laser will produce a diffuse beam with substantially 30 degree FWHM. Conventional diffusers often have a symmetric, having the FWHM in all azimuthal orientations, while some diffusers may have an elliptical light distribution pattern, having one FWHM in a first azimuthal orientation, and a substantially different FWHM in a second azimuthal orientation substantially perpendicular to the first. Many other diffusion patterns are also known in the art.